It is commonly known that certain metal alloy materials are difficult to connect to one another by welding. In particular, laser welding of certain materials are known to exhibit hot cracking, micro-cracking, and/or enbrittlement along the weld joint. This cracking typically has a tendency to move along the center of the weld nugget and is evident not only in regular laser welding, but also in the other traditional welding processes, such as gas metal arc and gas tungsten arc, when filler metals are not used. These processes all use a high energy heat source to melt the metal and form a molten metal pool. The point of resolidification is just after the weld pool. The middle of the weld joint is typically subjected to severe thermal gradients. The heat is quickly conducted to the colder regions of the material being welded. These severe thermal gradients cause a fast quench in the material being welded, which in turn causes minute fissures, or cracks, to form along with elements such as silicon in the case of 6000 series aluminum alloys. Silicon or carbon are part of the metallurgical compositions of certain aluminum or steel alloys. The carbon or silicon emerge in their elemental form during welding and further weaken the weld.
A process has previously been proposed for laser welding a cover and a casing each made of an aluminum or an aluminum-based alloy, via a nickel layer plated thereon, thereby forming a package for electronic devices. The process includes boring apertures in the sides of the casing through which input and output terminals can be inserted and soldered hermetically to the casing, plating a nickel layer on the surface of the casing, mounting electronic devices in the casing and forming necessary connections between the devices and the terminals, welding the cover and the casing together by pulse YAG laser irradiation to form a weld zone containing 1.5 to 10.0% of nickel by weight and sealing the cover and the case hermetically.
Another method has been proposed of welding together aluminum alloy workpieces of the same aluminum alloy. This method includes the steps of disposing a selected alloying metal on the workpiece in the desired weld areas, locally heating the workpieces in the welding area to form a localized liquid solution, and solidifying the localized liquid solution to form the weld between the like aluminum alloy workpieces. The selected alloying metal is silicon for aluminum alloys of interest, and a laser is the preferred heating means for effecting the welding.
A method for forming an alloy layer on an aluminum alloy substrate by irradiating with a CO.sub.2 laser has also been proposed. In this method, a powder for alloying, containing a substance to be alloyed with the substrate and an element selected from the group consisting of silicon and bismuth, is disposed on the surface of the aluminum alloy substrate. The powder is then irradiated with a CO.sub.2 laser, so as to be melted and fused together with a surface portion of the aluminum alloy substrate, so that these two are alloyed together.